Well, not to be rude, but you've definitely contradicted yourself here. If
you're familiar with the differnces between RGB and CMYK, between an
additive colorspace and a subtractive colorspace, between a projected
colorspace and a reflective colorspace, how can you go on to say that you
expect an RGB scan of a photo which is then converted to CMYK to print the
same as the original? It's gone through a process that can't really print
all of the colors because they're outside the range of the colormodel.
You might want to read up on the tips here: www.scantips.com and
www.retouchpro.com may have some info for you, too.
Some may depend on the color profile you may or may not be working with. Are
you sending this out to be printed or are you trying to use a home-use ink
jet printer? If inkjet do you have 4 separate cartridges for colors or just
2? Have you considered getting the new 6 and 7 color printers? Some actually
have a "blue" and "red" cartridge to help with RGB images (allegedly). Are
you using photo paper or plain paper? Glossy or matte? Also, what dpi/ppi
level are you scanning at? I'm not saying I have the answers for these
things... But they're items to look at.
Now if you're converting things to a single color, you could always look at
Pantone color systems.